European Patent Applications 0 501 626 and 0 501 627 disclose an apparatus of the generic type, having means for delivering, depositing, aligning, and holding down individual sheets in a collecting tray to form a sheet stack from sheets collected one on top of another, in which the alignment means are continuously in engagement with the topmost sheet of the stack during sheet delivery. This leads on the one hand to increased wear on a rotating alignment roller provided in the alignment means, and on the other hand can result in high loading of the topmost sheet, such as curling, creasing, or lifting of the sheet at the delimiters.
These disadvantages cannot be entirely eliminated either by the reduction made in the contact force of the alignment roller and of the holddown element, or by the specific physical configuration of the holddown element. In addition, the low contact force of the alignment roller and of the holddown element results in unreliable edge alignment and flatness of the topmost sheet. Moreover, it is not possible for sheets to be properly delivered, stacked, and aligned in a collecting tray having a depositing surface which rises in the delivery direction, since transport and positioning of the sheets cannot be guaranteed by the reduced friction of the alignment roller and the low holding force of the holddown element.
The reduction in contact force is accomplished, in this context, by slaving the alignment roller and holddown element as a function of the stack height by means of a motor-driven drive unit and an additional linear stroke spring device.
German Patent No. 42 07 070 discloses an apparatus having means for delivering, depositing, and aligning individual sheets in a collecting tray to form a sheet stack from sheets collected on top of one another, in which the alignment means are lifted while the topmost sheet of the stack is being delivered, and is only then lowered for alignment onto the topmost delivered sheet of the stack.
In this aforesaid apparatus, the alignment means are lowered by means of a controlled linear stroke electromagnet from the upper initial position, sensed by a sensor, in which a completed sheet stack can be removed, into the lower end position or onto the stack. In addition, two rotating alignment rollers of the alignment means are driven by means of a motor-driven drive unit.
It is disadvantageous that on the one hand, the sheet stack is not securely held by means of a holddown element before and during sheet delivery, so that for the purpose of reliable operation (as disclosed), only one collecting tray with a depositing surface sloping down in the delivery direction, and with a front delimiter attached at the lower end, can be used; and on the other hand that an additional drive unit for the alignment rollers is required.
It is therefore the object of the invention to create an apparatus of the generic type which on the one hand prevents high wear on the alignment rollers and high loading of the topmost sheet of a sheet stack, while at the same time reliably aligning the sheets delivered onto the stack and, in addition, securely holding the sheet stack in the collecting tray, and which on the other hand allows easy removal of a completed sheet stack from the collecting tray, and has a simple and compact design, this guaranteeing functionality in an automatic environment.